Although this isn't one of the hot topics right now (amazing how gay marriage was no longer an Urgent! Threat! after the election), Jim Henley has a well-written and well-argued post up that takes apart at least one conservative argument against gay marriage. It's works at the level of social structure and policy rather than moral values—the kind of social policy argument I prefer, particularly on this issue where the moral argument is unlikely to sway anyone. Either you think homosexuality is immoral or you don't. Since I don't (and think that the position that demonizes homosexuality is, itself, immoral), any argument against gay marriage on that basis is sure to bounce off my evil, liberal armor. The secular, pragmatic arguments Henley treats are, thus, far more interesting.
> "I think that the position that demonizes homosexuality is, itself, immoral"
Can you explain your position here a little more? I agree with you WRT homosexuality in general, but if someone disagrees with homosexuality on religious grounds how is that immoral?
It's not in their moral system. It is in mine. I find it immoral as it tends to make homosexuals an Other, a Them that is fundamentally different from normal humans, thus dehumanizing and objectifying homosexuals. The mental structure that posits homosexuality as immoral works the same as racism or sexism: there is a certain class of people that is distinctly and irrevocably different. While many will argue that they hate the sin of homosexuality, not homosexuals themselves, in practice it does not work that way. It's like saying it's black skin you hate, not blacks. Of course, this last analogy assumes that homosexuality is an intrinsic part of oneself, an assumption that many (most? all?) who oppose gay marriage disagree with. I, however, think it is a safe and sane assumption based on scientific, psychological, sociological, and anecdotal evidence as well as personal experience.